Cryogenic Rearrangements of Spiroheptadiyl: Light- or Heavy-Atom Quantum Tunneling?

Triplet 1,4-spiro[2.4]heptadiyl (SHD) has been shown experimentally to undergo rapid ring-opening and subsequent 1,2-hydrogen shift upon generation via photolysis of a diazene precursor at cryogenic temperatures. Modern computational tools elucidate the potential energy surface and kinetics behind this cascade reaction, disproving the earlier hypothesized mechanism invoking hot molecule effects in the first ring-opening step and tunneling in the second hydrogen transfer step. Instead, our results assign the SHD instability to heavy-atom tunneling and a subsequent photochemical hydrogen shift. This essentially is the opposite of the originally proposed mechanism. This case study thus addresses common misconceptions about the fundamental principles of tunneling involving hydrogen or carbon.